In a conventional vehicle engine, a cylinder compression ratio (CR) is fixed, with a piston moving between a consistent top-dead-center (TDC) and bottom-dead-center (BDC) during each combustion cycle. If the CR is set at a low ratio to deliver maximum power during engine operation, the low CR may result in undesirable combustion of excess fuel during light engine loads and speeds. Conversely, if the CR is set at a high ratio to prioritize fuel efficiency, a power output of the engine may be degraded when increased torque is requested.
To mitigate the above issues, an engine may be adapted as variable compression ratio (VCR) engine and equipped with various mechanisms to alter (e.g., mechanically alter) a volumetric ratio between the piston TDC and BDC. Thus the CR may be adjusted as engine operating conditions change. As a non-limiting example, a VCR engine may be adapted with a retrofit VCR system that includes a mechanical piston displacement changing device (e.g., an eccentric) that moves the piston closer to or further from the cylinder head, thereby changing the size of the combustion chambers. Still other engines may mechanically alter a cylinder head volume. The retrofit VCR system may enable reconfiguration of an engine with a fixed compression ratio to have an adjustable compression ratio that is varied according to engine operations, thereby increasing vehicle fuel economy.
In some retrofit VCR systems, the eccentric used to alter the piston position may be controlled by a gear system. The gear system may rotate, and, in turn, rotate the eccentric, leading to variations in piston height. However, friction between components of the gear system may generate undesirable sounds while the gear system rotates, leading to noise, vibration and harshness (NVH) issues. Furthermore, an actuator driving movement of the gear system may be costly, occupying space in an already space-restricted compartment and adding weight to the engine.
One example approach to address the NVH and actuator issues is shown in Chinese Patent Application No. CN 205638695. Therein, a VCR system includes a rod assembly connected to a piston, movement of the rod assembly actuated by an eccentric bushing. The eccentric bushing is coupled to an end of the rod assembly proximate to the piston and distal to a crankshaft. A height of the piston is adjusted by the eccentric bushing and a position of the piston is maintained by a hydraulically actuated locking pin. The engine is adjusted between a low compression ratio state and a high compression ratio state by activation of an oil pump when the piston is at TDC, generating hydraulic pressure in an oil chamber that counters an elastic force of a compression spring, thereby pulling the locking pin out of a first locking hole in the eccentric bushing. The eccentric bushing is allowed to rotate until the piston is at a desired height. The oil pump is deactivated, dissipating the hydraulic pressure and allowing the compression pin to slide the locking pin into a second locking hole. When the locking pin is inserted into the second locking hole of the eccentric bushing, a position of the eccentric bushing and the piston height is maintained. The compression ratio is readily varied between a high ratio and a low ratio without relying on a complex mechanical actuation system.
However, the inventors herein have recognized potential issues with such systems. As one example, coupling the eccentric bushing to the end of the conrod coupled to the piston positions the eccentric bushing at a small end, e.g., smaller than an opposite end, of the conrod. Addition of the eccentric bushing to the small end of the conrod adds reciprocating weight that may lead to mass imbalance during rotation of a crankshaft. Imbalance of masses at the crankshaft may contribute to NVH issues, particularly as high engine speeds. To counter forces resulting from the reciprocating weight, one or more balance shafts may be added to the engine, or, if the engine already has a balance shaft, a size of the balance may be increased. The one or more balance shafts may generate friction that consumes fuel energy to overcome, offsetting fuel economy benefits obtained through implementation of the VCR system and increasing a cost, complexity, and weight of the engine.
In one example, the issues described above may be addressed by a method for a variable compression ratio (VCR) mechanism, including an eccentric with a first detent and a second detent, the first and second detents arranged on opposite faces of the eccentric and positioned 180 degrees relative to one another around a circumference of the eccentric, the eccentric configured to be adjusted between a locked position and an unlocked position, a first locking pin configured to be inserted into the first detent of the eccentric and housed in a first oil chamber, a second locking pin configured to be inserted into the second detent of the eccentric and housed in a second oil chamber, and a valve fluidly coupled to the first oil chamber and the second oil chamber.
In this way, the engine compression ratio may be varied without modifications to the engine block and without inducing NVH issues or implementation of a costly and complex system to actuate adjustment of the compression ratio and provide mass balance to the engine.
As one example, a VCR engine may include eccentrics rotatably coupled to crank pins of a crankshaft. Each of the eccentrics is connected to a piston by a conrod, arranged at an end of the conrod distal to the piston, and extending between the eccentric and a base of the piston. The eccentrics may be adapted with a first slot and a second slot, configured to mate with a first locking pin and a second locking, respectively. The eccentrics may be alternatively locked in a first position by engagement of the first locking pin with first slot or a second position by engagement of the second locking pin with the second slot. The first position and second position correspond to different piston heights, and therefore, different compression ratios. Movement of the locking pins is controlled by hydraulic pressure provided by oil reservoirs, thereby enabling adjustment of the compression ratio using a simple system that does not produce NVH effects or cause mass imbalance in the engine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.